There has been already proposed a frequency converter including a power supply side converter for converting an a.c. power delivered from an a.c. power supply through a reactor to a d.c. power, and a motor side converter for delivering a variable voltage and variable frequency a.c. power to an a.c. motor, both converters being comprised of a voltage-source PWM (Pulse Width Modulation) converter, d.c. terminals of the both converter, d.c. terminals of the both converters being connected to each other, a smoothing capacitor being connected between positive and negative terminals of the both converters. This frequency converter can attain an improvement in a power-factor when viewed from an a.c. power supply, i.e., power-factor on the side of power supply, and can carry out delivery and receipt of a power during power running of a motor and a power during regenerative running thereof.
When a load current flows in a frequency converter of this kind, a voltage drop V.sub.L is produced in a reactor connected between the a.c. power supply and the power supply side converter. The power supply side converter must compensate such a voltage drop as well to perform a control operation so that its output voltage becomes equal to a predetermined value. Assuming that a power-factor on the power supply is designated by .PHI. and the relationship expressed as .PHI..apprxeq.1 holds, this voltage drop V.sub.L is produced at a phase lagging by an angle of 90 degrees with respect to the power supply voltage. When an attempt is made to compensate voltage drop V.sub.L produced at a phase lagging by an angle of 90 degrees by using the power supply side converter to control its output voltage so that it is equal to a predetermined value, the control circuit for the power supply side converter would have to handle a larger control signal as a vector. Particularly in the case where a load current is large, since the reactor voltage drop V.sub.L also become large, the control signal also becomes large, resulting in the possibility that the control signal exceeds above the linear control range of the control circuit to reach the saturation range. When the operation mode of the control circuit has reached the saturation range, there occurs an extraordinary state in control such that a d.c. voltage on the power supply side converter is lowered during power running and is raised during regenerative running.